This invention relates to the art of producing thin-walled honeycomb structures from extrudable material such as particulate ceramic and/or metal batches and similar materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity. More particularly, the present invention relates to an improved extrusion die and mask assembly for forming a honeycomb structure with a smooth or contoured integral outer peripheral skin, wherein the usual distortion of the peripheral cells of the extruded honeycomb structure is virtually eliminated.
The prior art is replete with extrusion die and mask assemblies of different configurations for providing cellular structures with an outer skin; however, virtually all of such assemblies produce some distortion within the outer peripheral cells of the extruded honeycomb structure, and do not contemplate the unique concerns of the extrusion die apparatus of the present invention.
U.S. Pat. No. 3,790,654 relates to method and apparatus for forming honeycomb structures with an integral casing. An adjustable insert, having a tapered surface, is provided for varying the resistance to flow of the batch material supplied to an annular orifice for forming the integral casing about the honeycomb structure.
U.S. Pat. No. 4,168,944 relates to a complex apparatus for manufacturing a tubular honeycomb assembly with an adiabatic layer formed integrally on the peripheral wall thereof. Like the previously mentioned '654 patent, the casings or pipe sections of the '944 patent are extruded from an annular orifice.
U.S. Pat. No. 4,298,328 relates to an extrusion apparatus for preventing the distortion of peripheral cells in an extruded honeycomb structure. The die body and the mask are provided with parallel walls converging in the extrusion direction to form a peripheral gap therebetween, which gap may be varied to produce honeycomb structures with differing skin thicknesses.
U.S. Pat. No. 4,368,025 relates to an extrusion device for producing honeycomb structures wherein the skin is formed outwardly of the pin face of the die, by grooves formed in an overlying die mask, or by an annulus formed in the pins themselves, with openings formed in the mask face.
U.S. Pat. No. 4,381,912 discloses an extrusion die for forming a honeycomb structure wherein skin material flows laterally through the peripheral discharge slots alone, or through such slots and a gap, to knit with center webs and form a peripheral skin thereon.
U.S. Pat. No. 4,349,329 relates to an extrusion device for producing honeycomb structures wherein a pooling zone supplies material to form an outer wall and thickened web portions between outer peripheral cells.
U.S. Pat. No. 4,668,176 relates to an extrusion device for the production of honeycomb structures wherein a spring element controls the thickness of a passageway for the flow of skin material, and accordingly the thickness of the resulting skin, and a pivotal plate controls the flow of such skin material to such passageway.
U.S. Pat. No. 4,710,123 sets forth an extrusion device for the production of honeycomb structures wherein the skin flow rate is adjusted by the pneumatic movement of a ring, and the thickness of the skin is determined by a bolt-adjusted sleeve member.
U.S. Pat. No. 4,814,187 relates to an extruding die apparatus for forming honeycomb structural bodies wherein the rate of flow of the extrusion material is controlled by the use of flow impedance plates to provide an even flow across the die face.
Japanese Unexamined Patent Application Publication 61-5915 relates to an extrusion die for forming honeycomb shape material, wherein the rate of extrusion of the material adjacent the skin is controlled by the use of shorter rate-adjustment pins so as to match the rate of extrusion at the center of the die.
In order to overcome the problems and complexities with the above-noted die assemblies of the prior art, it is an object of the present invention to provide an extrusion die assembly for forming thin-walled cellular substrates with a smooth or uniformly contoured skin, such that the webs of the outer peripheral cells of the substrate are not detrimentally deformed or distorted during the skin formation, as was prevalent in the past.
Operationally, improved dry compressive strength of the skin-covered extruded cellular substrate is obtained by reason of the fact that the webs forming the peripheral cells of the substrate are virtually distortion free, due to the fact that detrimental lateral or transverse flow of the skin-forming material, which causes distortion, is virtually eliminated.